## **Core Concept**
Cohort studies are a type of observational study used in research to understand the development of outcomes over time. They involve following a group of individuals who share a common characteristic or experience within a defined period. The main purpose is to identify potential causes of a disease or outcome by observing exposures and outcomes over time.

## **Why the Correct Answer is Right**
Cohort studies are particularly useful for estimating **incidence** (the number of new cases that develop in a specified time period among a population at risk), **relative risk** (the ratio of the probability of an event occurring in the exposed group versus a non-exposed group), and **risk ratio** (similar to relative risk, comparing the risk of a certain event (or of developing a certain outcome) between an exposed group and a non-exposed group). However, cohort studies are not primarily designed to estimate **prevalence**, which refers to the total number of cases of a disease in a population at a given time. Prevalence studies are more commonly associated with cross-sectional studies.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Cohort studies can estimate **incidence**. This is one of the strengths of cohort studies; they can directly measure the rate of occurrence of new cases of a disease.
- **Option B:** Cohort studies can estimate **relative risk**. By comparing the risk of developing a disease or outcome between exposed and non-exposed groups, cohort studies can directly calculate relative risk.
- **Option C:** Cohort studies can estimate **risk ratio**. The terms risk ratio and relative risk are often used interchangeably, and cohort studies are well-suited to calculate this measure.

## **Why Option D is Correct but Explained Here for Completeness**
- **Option D:** Cohort studies are not primarily used to estimate **prevalence**. Prevalence is the total number of cases (old and new) of a disease in a population at a given time and is typically measured through cross-sectional studies.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that cohort studies are useful for studying **rare exposures** and can establish **temporality** (cause precedes effect) well, which is crucial for inferring causality. However, they can be **time-consuming** and **expensive**, especially for long-term exposures or outcomes.

## **Correct Answer: D. Prevalence**